The invention relates to a method for heating up bulk materials, in particular granular plastic material.
German Patent Specification DE 31 31 471 discloses a method in which bulk material is heated up and dried in a drying vessel. For that purpose, chambers are provided which are filled with adsorbing agents. The dry air is fed to the drying vessel in a closed circuit; wherein the chambers are regenerated by a second air circuit at time intervals. In order to dry the bulk material, the dried air is heated, passed through the bulk material, and it is then dried again.
In addition, two drying methods for drying granular material are described in the special edition xe2x80x9cMASCHINENMARKTxe2x80x9d (=xe2x80x9cMachines on the Marketxe2x80x9d), 81st year, vol. 90, dated Nov. 11, 1975. The first of these involves using hot-air driers; the second involves using dry-air driers. Hot-air driers are not suitable for drying hygroscopic granular material, because, in this case, the drying process depends upon the humidity of the ambient air. In addition, the energy-balance of the hot-air driers is negative, because the heat contained in the outgoing air is lost to the drying process. In principle, bulk material driers of this kind are also suitable for heating up granular plastic material. However, the disadvantage of such bulk-material drying plants is that the residence time of the bulk material in the drying vessel is relatively long and thus there is a danger that the bulk material, in particular granular plastic material, will be thermally damaged.
It is therefore the object of the present invention to avoid the aforementioned disadvantages and to provide a process for heating bulk material, especially granular plastic material, which treats the material gently and which can be used in an economical way.
This object is achieved by the invention as described and claimed hereinafter.
The present invention has the advantage that a vessel is selected for the bulk material, and the bulk material is passed through the vessel in a quasi-continuous manner. Depending on the granular size of the bulk material, a minimum residence time in the vessel is required in order to thoroughly heat the material to the core. Based on calculations, the theoretical residence time, for example, with conventional granular plastic materials amounts to a few seconds, so that it is advantageous to strive for a residence time of approximately 2 to 5 minutes.
In accordance with the invention, various alternative processes are described herein. It is thus advantageous to guide the material flow and the heat-carrying gas flow in parallel and to vary the gas inlet temperature in such a way that the material outlet temperature corresponds to the required temperature. It is likewise possible to vary the quantity of gas instead of the gas inlet temperature, so that the material outlet temperature corresponds to the required temperature.
The process for heating bulk materials according to the present invention is characterized by the fact that the heat-transfer gas flow and the final temperature of the material to be heated are not thermally balanced. Therefore, the temperature of the gas flow is substantially higher than the final temperature of the material to be heated.
As an alternative to the processes described above, the heat-transfer gas flow can be conveyed countercurrently to the direction of the material flow. In this case there are again two practicable variations of the process. The first is to control the gas inlet temperature, and the second is to control the quantity of gas in such a way that the material outlet temperature corresponds to the required temperature.
Another embodiment of the invention envisions varying the gas temperature or the quantity of gas in the exhaust of the heat-transfer gas, i.e. in the exhaust air duct for the heat-transfer gas, by means of a temperature sensor provided for measuring the temperature of the exhaust air, so that the temperature of the material can be determined from the measured temperature.
A further embodiment of the invention envisions determining the control variable for varying the gas temperature or quantity of gas by means of a material temperature sensor which is disposed in a lower region of the vessel through which the gas does not flow.
To optimize the overall process and to avoid heat losses from the material, another embodiment of the invention envisions providing the vessel with a so-called dynamic insulation. This is achieved by guiding the outgoing process air, i.e. the exit air, over the outer surface of the vessel. For this purpose, the vessel is provided with a second, outer jacket.
These and other features of preferred embodiments of the invention, in addition to being set forth in the claims, are also disclosed in the specification and/or the drawings, and the individual features each may be implemented in embodiments of the invention either individually or in the form of subcombinations of two or more features and can be applied to other fields of use and may constitute advantageous, separately protectable constructions for which protection is also claimed.